IEFBR14

IEFBR14 is an IBM mainframe utility program. It runs in all IBM mainframe environments derived from OS/360, including z/OS. It is a placeholder whose purpose is to do nothing. As it turned out, over the years, its attempt to do nothing was too concise and would cause problems with related tools, leading to the slight expansion of the program.

Purpose

Allocation

On OS/360 and derived mainframe systems, most programs never specify files (usually called datasets) directly, but instead reference them indirectly through the Job Control Language (JCL) statements that invoke the programs. These data definition (or "DD") statements can include a "disposition" (DISP=...) parameter that indicates how the file is to be managed — whether a new file is to be created or an old one re-used; whether the file should be deleted upon completion or retained; etc.

IEFBR14 was created because while DD statements can create or delete files easily, they cannot do so without a program to run. The program used in the JCL does not actually need to use the files to cause their creation or deletion — the DD DISP=... specification does all the work. Thus a very simple do-nothing program was needed to fill that role.

IEFBR14 can thus be used to create or delete a data set using JCL.

Deallocation

A secondary reason to run IEFBR14 was to unmount tapes that had been left mounted from a previous job, perhaps because of an error in that job's JCL or because the job ended in error. In either event, the system operators would often need to demount the tapes, and a started task – DEALLOC – was provided for this purpose.

Simply entering the command

S DEALLOC

at the system console would run the started task, which consisted of just one step

//STEP01    EXEC PGM=IEFBR14

Naming

The "IEF" derives from a convention on mainframe computers that programs supplied by IBM were grouped together by function or creator and that each group shared a three-letter prefix. In OS/360, the first letter was almost always "I", and the programs produced by the job management group (including IEFBR14) all used the prefix "IEF". Other common prefixes included "IEB" for dataset utility programs, "IEH" for system utility programs, and "IEW" for program linkage and loading.

As explained below, "BR 14" was the essential function of the program, to simply return to the operating system. This portion of a program name was often mnemonic — for example, IEBUPDTE was the dataset utility (IEB) that applied updates (UPDTE) to source code files, and IEHINITT was the system utility (IEH) that initialized (INIT) magnetic tape labels (T).

As explained further in "Usage" below, the name "BR14" comes from the IBM assembler-language instruction "Branch (to the address in) Register #14", which by convention is used to "return from a subroutine". Most early users of OS/360 were familiar with IBM Assembler Language and would have recognized this at once.

Usage

Example JCL would be :

//IEFBR14  JOB  ACCT,'DELETE DATASET',MSGCLASS=J,CLASS=A
//STEP0001 EXEC PGM=IEFBR14                       
//DELDD    DD DSN=xxxxx.yyyyy.zzzzz,
//            DISP=(MOD,DELETE,DELETE),UNIT=DASD

To create a Partitioned Data Set:

//TZZZ84R  JOB NOTIFY=&SYSUID,MSGCLASS=X                    
//STEP01    EXEC PGM=IEFBR14                                 
//DD1       DD DSN=TKOL084.DEMO,DISP=(NEW,CATLG,DELETE),           
//             DCB=(RECFM=FB,LRECL=80,BLKSIZE=80,DSORG=PO),  
//             SPACE=(TRK,(1,1,1),RLSE),                       
//             UNIT=SYSDA

Implementation

IEFBR14 consisted initially of a single instruction a "Branch to Register" 14. The mnemonic used in the IBM Assembler was BR and hence the name: IEF BR 14.

The linkage convention for OS/360 and its descendants requires that a program be invoked with register 14 containing the address to return control to when complete, and register 15 containing the address at which the called program is loaded into memory; at completion, the program loads a return code in register 15, and then branches to the address contained in register 14.

The original version of the program didn't alter register 15 at all. Since the program was not loaded at address zero (as that address range is reserved for hardware use), the return code was not zero. Hence, a second instruction had to be added to clear the return code so that it would exit with the correct status.

The machine code for the modified program is:

        SR    R15,R15  put zero into register 15 (return code)
        BR    R14      branch to the address in register 14 (return to scheduler)

History from the RISKS Digest

Here is an article contributed by John Pershing to the RISKS Digest[1] that explains some of the history of IEFBR14. Note that there is an error in the final version of the code as posted—it contains the instruction BR GO, however GO is an instruction label, not a register. The correct code would be B GO. As of November 2009, the code is once again the very brief second version.

From: John Pershing <PERSHNG@ibm.com>
Date: 25 Jan 88 11:41:42 EST

You can't even necessarily write the null program without encountering
problems...

There is an apocryphal story about the large number of attempts that were
required in order to produce a "correct" version of MVS's null program,
IEFBR14 (this was done back in the days when MVS was still called OS).
As with all MVS programs, IEFBR14 is called using the standard system
calling conventions, and all it has to do is return successfully.

The first version was something like this:

         IEFBR14 START
                 BR    14       Return addr in R14 -- branch at it
                 END

First bug:  A program indicates its successful completion by zeroing
register 15 before returning; this version of the null program "failed"
every time.  Try it again:

         IEFBR14 START
                 SR    15,15    Zero out register 15
                 BR    14       Return addr in R14 -- branch at it
                 END

Much better.  However, this caused some-or-other problems with the linkage
editor, since the END statement didn't specify the primary entry point
of the routine.  Version three:

         IEFBR14 START
                 SR    15,15    Zero out register 15
                 BR    14       Return addr in R14 -- branch at it
                 END   IEFBR14

At least now, the null program was functionally correct.  However, dump
analysis was impeded because the program didn't include its own name in
the source code, as an "eyecatcher" (this is a time-honored convention).
Null program, mark four:

         IEFBR14 START
                 USING IEFBR14,15  Establish addressability
                 BR    GO          Skip over our name
                 DC    AL1(L'ID)   Length of name
         ID      DC    C'IEFBR14'  Name itself
                 DS    0H          Force alignment
         GO      SR    15,15       Zero out register 15
                 BR    14          Return addr in R14 -- branch at it
                 END   IEFBR14

The next change had something esoteric to do with save-area chaining
conventions -- again, for the sake of conventions and to keep the dump
analysis tools happy.

Note that the "null program" has tripled in size:  both in terms of the
number of source statements and in terms of the number of instructions
executed!

See also

References

Trombetta, Michael & Finkelstein Sue Carolyn (1985). "OS JCL and utilities". Addison Wesley. page 152.

  1. ↑ Pershing, John (1988-01-25). "Safe programming languages". RISKS Digest. Retrieved 2006-10-12.

External links

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